The present invention relates to a cooled air cycle system and to a method for operation such a system.
Refrigeration systems usually employ a two-phase cycle with a refrigerant that alternates between gaseous and liquid states. Such systems have a large cooling capacity due to the amount of heat that can be taken up when the refrigerant evaporates and are therefore superior to any alternative in most applications, normally also for air-conditioning. However, the equipment in such a system is circumstantial and expensive and requires a closed piping circuit to isolate the usually pollutional refrigerant from the environment. And since refrigeration systems at the end transfer the cold through air or another gas, these conventional systems incorporate both a primary cooling medium, i.e. the refrigerant in the closed circuit and a secondary cooling medium, i.e. the air that transfers the cold to the object to be cooled.
It has been known that refrigeration also can be attained by a cooled air cycle system, also called open reversed Brayton cycle. In such a system the air itself is the working medium in the refrigeration cycle. There is thus no need for a special refrigerant that is cyclically condensed and evaporated and there is no need to transfer the cold from a refrigerant to the air, since they are one and the same. Such systems therefore also eliminate the risk for pollution. In a cooling system according to this principle the gas, normally air is first compressed, whereby the pressure and the temperature rise. The warm compressed gas is then cooled in a heat exchanger and thereafter expanded with substantially the same pressure ratio as that of the compressor. The air leaving the expander will be cooler than the air entering the compressor and can perform its cooling purpose, e.g. air-conditioning, which normally takes place at atmospheric pressure, i.e. the same pressure as the compressor inlet pressure if located at ground level.
Examples of system operating according to the cooled air cycle are disclosed in U.S. Pat. No. 3,045,447, U.S. Pat. No. 3,686,893, U.S. Pat. No. 3,965,697 and German DE 42 18 299.
Cooled air cycle systems have small cooling capacity in comparison with conventional cooling systems, which is the main reason why they rarely have come into use. For some applications, however, where conventional cooling systems due to environmental and other reasons are not, a cooled air cycle system might be an advantageous alternative. The low efficiency of such a system, however, calls for improvements in this respect in order to make the cooled air cycle system more competitive, both for cooling applications e.g. air conditioning and for heat pump applications.
The object of the present invention thus is to improve the efficiency of a cooled air cycle system.